Real Madrid striker picked an injury in the first-half of Tuesday’s 3-0 Champions League victory over Celtic.
Courtois said: “We don’t know much about it. When I picked up the ball from a corner, I sent it up to him quickly and that was when he felt something in the knee.
“I don’t think it’s anything serious because Karim would rather come off if he feels a twinge. I hope it’s nothing but we have other players who can operate in that position.”